Current issue is a bit of a 'tis/t'isnt on whether brake fluid boiling in, say the rear brakes is likely to kill the master cylinder (I assume by transferred heat and/or pressure).

I say it isn't likely, but I don't know this for a fact, and may be suffering from wishful thinking.

Here's the most relevant bit of the thread, snipped from some corner of a foreign website.

"Alex1000 wrote:IMHO. If boiling is started somewhere (assume in a rear wheel) soon it will happen everywhere in whole breaking system."

Can't see that. It'll boil locally, where the heat is. As the steam expands up the brake pipes, it'll cool and re-condense. If you kept driving and feeding more heat into the system eventually you'd heat it all up and I suppose the steam would travel the long, long, narrow, heat dissipating metal pipe to the master cylinder, but meanwhile, you've had a brake failure, and either:-

(a) scared the Ooops out of yourself and stopped.

(b) scared the Ooops out of yourself and crashed.

And stopped.

Either way you ain't cookin' no master cylinder no more, no how.

EDIT: I suppose sustained driving without using the brakes could get you to this situation, but I'd think that isn't very likely to happen (at least off-freeway) in Taiwan. ENDEDIT

The "oops" is of course courtesy of this sites "Mary Whitehouse" module. Forumosa isn't so fussy.

I find I can sometimes bypass the auto-censor on American websites by using British obscenities (e.g arse rather than ass).

Don't see why brake fluid boiling would cause master cylinder failure. The fluid will boil down at the caliper end, which might cause failures locally to the heat source, but up at the master cylinder? Weak theory to say the least. However, the extended piston travel in the master cylinder (resulting from boiled fluid) could damage its seal. As much has been reported in brake bleeding operations.

I didn't read the whole post (attention span not up to it), but I think the woman is full of it. She parked like her stereotype and is now attempting to save face. What she may not realise (due to lacking mechanical knowledge) is that her claim can be falsified. It would have to be bad fluid (which there would be evidence of), fluid leak (which there would be evidence of), or brake bind (which there would be evidence of).

Don't see why brake fluid boiling would cause master cylinder failure. The fluid will boil down at the caliper end, which might cause failures locally to the heat source, but up at the master cylinder? Weak theory to say the least. However, the extended piston travel in the master cylinder (resulting from boiled fluid) could damage its seal. As much has been reported in brake bleeding operations.

I didn't read the whole post (attention span not up to it), but I think the woman is full of it. She parked like her stereotype and is now attempting to save face. What she may not realise (due to lacking mechanical knowledge) is that her claim can be falsified. It would have to be bad fluid (which there would be evidence of), fluid leak (which there would be evidence of), or brake bind (which there would be evidence of).

Yeh, that thread does go on a bit, and contains some fairly obvious misinformation.

I'm inclined to agree re pilotette error, but I'm not certain, since I've had a fairly recent foot-to-floor brake failure myself, which is why I'm contributing (positively or negatively) to that thread.

I THINK mine was due to (old) boiling brake fluid, but again I'm not certain. Specific uncertainties are:-

(a) Could a master cylinder fault be intermittent, so it recovered "spontaneously"? My brakes seem OK now (after fluid, shoe and wheel cylinder replacement) but might there be a lurking gotcha? Opinions seem to vary.

(b) What of dual-circuit brakes? Whatever the cause, in my failure I didn't seem to have any residual braking. Does that point to the master cylinder?

I take your points re residual evidence in the alleged Mazda failure, with the caveats that:-

(a) I wouldn't necessarily be confident that mechanics here would spot it or retain it.

(b) If its true that Ford (and by extension Mazda) don't have brake fluid replacement in their service schedule, they'd have a powerful incentive (especially powerful for a US company) not to find (and/or lose) any evidence of old brake fluid involvement in potentially (and, its claimed in one of the linked articles, actually) fatal accidents.

I THINK mine was due to (old) boiling brake fluid, but again I'm not certain. Specific uncertainties are:-

(a) Could a master cylinder fault be intermittent, so it recovered "spontaneously"? My brakes seem OK now (after fluid, shoe and wheel cylinder replacement) but might there be a lurking gotcha? Opinions seem to vary.

I'm going to say no. Can't imagine how that would work. I've certainly never heard of or experienced it.

edlithgow wrote:

(b) What of dual-circuit brakes? Whatever the cause, in my failure I didn't seem to have any residual braking. Does that point to the master cylinder?

Could indicate master cylinder, were it not for the fact that the brakes recovered without any master cylinder work. Points to bad fluid (the only other common denominator).